To prevent corrosion of a metal component, a surface of the metal component is often plated and further coated with a chromate film. A colored chromate film as one of such chromate films is used in a wide field because the colored chromate film is excellent in corrosion resistance. A lot of chromate films excellent in corrosion resistance as well as the colored chromate film have not been, however, used intentionally from the point of view of environmental protection because they have a tendency to contain hexavalent chrome. Hereinafter, a chromate film substantially containing hexavalent chrome is also referred to as “hexavalent chromate film”.
To solve this problem, the present inventors have developed a chromate film substantially containing no hexavalent chrome but containing trivalent chrome as a large part of chrome component. Hereinafter, such a chromate film is also referred to as “trivalent chromate film”. The present inventors have succeeded in keeping corrosion resistance good by making the chromate film thick. As described above, use of the trivalent chromate film in place of the hexavalent chromate film has been advanced as use of hexavalent chrome has been substantially limited in the world.
On the other hand, the trivalent chromate film is often applied on a metal component for a plug such as a spark plug or a glow plug. Corrosion resistance at a temperature higher than ordinary temperature becomes a subject of discussion because such a plug is used after attached to an internal combustion engine. The present inventors have developed a trivalent chromate film having corrosion resistance kept good even at a high temperature and have prepared the spark plug or glow plug having such a chromate film formed thereon as disclosed in JP-A-2000-252042 (the term “JP-A” as used herein means an “unexamined published Japanese patent application”) JP-A-2000-249340, etc.